Funny enough, if you consult most of the companies' sites listed in the previous replies, all the top kits include slotted discs in order to prevent gas buildup between the pad and the disc during hard braking. They also claim that it helps remove glazing from the pad surface after hard use of the brakes.
The last one I can attest to : after numerous problems with the original discs of my '02 Acura TL-S (go visit any TL-S site: transmission and brakes are the two common problems with that car), I changed the front OEM discs to a set of Viper slotted/perforated discs front, and never had a problem anymore. I also changed the pads, but I can't remember the brand - kept the original front calipers. I overheated my front brakes (I won't tell you how and why, but let's say it was not what Acura had in mind when they built that car!), they were really glazed, but the "no brake" feel disappeared by itself by what I think was just regular braking, this within two days without having to remove the darn pads to sand them.